1. Field of the Invention
This invention relates generally to machine tools and is concerned more particularly with machine tools of the type known as "multi-spindle chucking machines".
A machine tool of this type includes a series of driven rotary spindles, each provided with a chuck or other device in which a workpiece can be held and presented to a stationary tool for machining. Multi-spindle machines are particularly useful where multiple machining operations are required to be performed on each of a succession of similar workpieces. A typical example of such a requirement is in the manufacture of automotive disc brake rotors, where each of a succession of cast rotor blanks must be machined at different speeds in forming the finished rotor.
2. Description of Prior Art
A typical prior art multi-spindle chucking machine for use in the manufacture of disc brake rotors includes a series of rotary spindles (typically two or three) mounted parallel to one another in a carrier and each provided at one end with a chuck which protrudes from the carrier for carrying a rotor blank to be machined. The spindles are driven at different but fixed speeds from a single drive motor through a mechanical gear transmission which allows the carrier to be indexed to present each blank to successive tools for machining at different speeds. The carrier is of generally cylindrical shape overall and is supported at its periphery in large diameter annular bearings which encircle the carrier. A splined locking arrangement is provided for retaining the carrier in a position to which it has been indexed.
Prior art machines of this type suffer from a number of disadvantages. For example, the fact that the spindles are driven at fixed speeds restricts the versatility of the machine. In some cases, it would be desirable to be able to vary the machining speed, for example, to take advantage of different cutting tools and/or to be able to accommodate different workpieces, e.g. disc brake rotors for different model cars. The fixed gear transmission arrangements of the prior art make this practically impossible.
Another disadvantage is that the large annular bearings required to support the carrier are extremely expensive. Not only does this make the initial capital cost of the machine high, but maintenance costs are high because the bearings tend to wear out. A related difficulty is that the bearing tolerances must be held to within fine limits because any radial movement of the carrier will have a detrimental effect on the accuracy of machining. These close tolerances in turn make the bearings vulnerable to ambient temperature variations which, in some circumstances, can cause the tolerances to diminish to the point at which the carrier becomes extremely difficult to index. This, in turn, means that high power capacity drives must be used in the indexing mechanism. A still further disadvantage is that the splined locking mechanism conventionally provided for the carrier of prior art multi-spindle chucking machines is extremely costly to manufacture and vulnerable to wear and jamming due to the presence of workpiece chips and other debris inevitably encountered in a production machine shop environment.
An object of the present invention is to provide a number of improvements in multi-spindle chucking machines aimed at addressing these problems.